A steam generator for a pressurized-water reactor power plant is usually constructed with a vertical cylindrical feed-water casing having a top closed by a steam dome provided with a steam outlet and a bottom closed by a tube sheet in which are mounted the inlet and outlet legs of an inverted U-shaped tube bundle, the bottom of the tube sheet having inlet and outlet reactor water-coolant manifolds for circulating the coolant through the tube bundle via these inlet and outlet legs. The tube bundle is surrounded by a cylindrical shroud spaced radially inwardly from the casing to define a descent space, the bottom of the shroud having an opening via which this descent space connects with the shroud's interior. The casing has a feed-water input inlet maintaining a supply of feed-water in the casing, the feed-water rising within the shroud while heating, flowing from the top of the shroud and descending in the descent space to again ascend within the shroud. The top of the shroud usually has a steam-water separator which separates the steam vaporizing from the ascending water within the shroud and discharging separated water back into the descent space. This descending water retains a substantial amount of its heat.
To avoid thermal shock and provide increased efficiency, a feed-water preheater surrounds the lower end of the tube bundle's outlet leg and connects with a feed-water supply pipe line supplied with feed-water of adequate pressure for introduction to the generator's casing. This preheater receives its heat from the heat exchanger tube bundle's outlet leg and discharges preheated feed-water inside of the shroud above the tube sheet, thus avoiding thermal shock and increasing efficiency.
The rate the feed-water is fed depends on the steam output demand of the steam generator, and if this is reduced, the feed-water input rate must also be reduced. The heat exchanger is heated by the pressurized-water coolant from the reactor providing the heat exchanger with a substantially constant heat input. If the feed-water input rate must be reduced, the feed-water receives an excessive amount of heat from the heat exchanger during the preheating, thus overheating the feed-water, causing the feed-water to boil, and the feed-water with boiling precipitating out compounds inevitably included by the feed-water and which are corrosive with respect to the preheater and heat exchanger.
The above shows that there is a problem concerning the provision of some kind of control of the rate of heat exchange effected in the preheater between the heat exchanger and the feed-water going through the preheater at a flow rate reduced from a normal rate for which the steam generator and its preheater are designed.